1. Field of the Invention
The present invention relates to a two-armed transfer robot useful for semi-conductor manufacturing equipment, liquid crystal display processing equipment and the like. More particularly, the present invention relates to a two-armed transfer robot for transferring workpieces between processing chambers under a vacuum
2. Description of the Related Art
In general, transfer robots are used for semi-conductor manufacturing equipment, liquid crystal display processing equipment and the like. The robot has at least one arm mechanism provided with a handling member. An object to be processed or workpiece such as a silicon wafer is placed on the handling member. The arm mechanism is capable of moving horizontally in a straight line as well as rotating in a horizontal plane. A plurality of processing chambers for performing various kinds of processing are arranged around a rotation axis of the robot. With the use of the transfer robot, the workpiece is suitably brought to and taken away from a selected one of the processing chambers.
For improving efficiency in the transferring operation, use has been made of the so-called two-armed transfer robot having two arm mechanisms. Each arm mechanism has a free end at which a handling member is mounted.
A conventional two-armed transfer robot is disclosed in Japanese Patent Application Laid-open No. 7(1995)-142552 for example.
Referring to FIGS. 14-17 of the accompanying drawings, the prior art robot includes a stationary base frame 80, an inner frame 81 and a first arm 82. The inner frame is rotatable about a vertical axis O.sub.1 relative to the base frame 80, while the first arm is rotatable about a first axis P.sub.1 extending in parallel to the axis O.sub.1. The rotation of the inner frame 81 is actuated by a driving device fixed to the base frame, while the rotation of the first arm 82 is actuated by a driving device fixed to the inner frame 81.
Reference numeral 83 indicates a second arm which is rotatable relative to the first arm 82 about a second axis Q.sub.1 extending in parallel to the first axis P.sub.1, while reference numeral 84A indicates a handling member which is rotatable relative to the arm 83 about a third axis R.sub.1 extending in parallel to the second axis Q.sub.1. Reference numeral 85 indicates a first rotation-transmittng member which is fixed to the inner frame 81 coaxially with the first axis P.sub.1, while reference numeral 86 indicates a second rolation-transmitting member which is fixed to the second arm 83 coaxially with the second axis Q.sub.1. Reference numeral 87 indicates a third rotation-transmitting member fixed to the first arm 82 coaxially with the second axis Q.sub.1, while reference numeral 88 indicates a fourth rotation-transmitting member fixed to the handling member 84 coaxially with the third axis R.sub.1.
A first connecting member 89 is provided between the first rotation-transmitting member 85 and the second rotation-transmitting member 86, while a second connecting member 90 is provided between the third rotation-transmitting member 87 and the fourth rotation-transmitting member 88. The distance S between the first and second axes is equal to the distance between the third and fourth axes. The radius ratio of the first rotation-transmitting member 85 to the second rotation-transmitting member 86 is 2 to 1. The radius ratio of the fourth rotation-transmitting member 88 to the third rotation-transmitting member 87 is also 2 to 1.
Chain sprockets or pulleys may be used for the first to fourth rotation-transmitting members 85-88. Correspondingly, the first and second connecting members 89, 90 may be chains or timing belts.
The first arm mechanism 91 is made up of the above-mentioned elements 82-90. A second arm mechanism 92, which is symmetrical to the first arm mechanism with respect to the X--X line, is supported for rotation about the second axis P.sub.2 extending in parallel to the axis O.sub.1.
Thus, the distance between the axis O.sub.1 and the first axis P.sub.1 is equal to that between the axis O.sub.1 and the second axis P.sub.2. The two-armed transfer robot is made up of the above elements 80-92.
The operations of the first and the second arm mechanisms 91, 92 are symmetrical with respect to the X--X line and substantially the same. Therefore, description will be made to the operation of the first arm mechanism 91.
First, it is assumed that the inner frame 81 is kept stationary to the fixed base frame 80, and that the first, second and third axes P.sub.1, Q.sub.1, R.sub.1 are initially located on a common straight line, as shown in FIG. 16. Starting from this state, the first arm 82 is rotated counterclockwise through an angle .theta. about the first axis P.sub.1.
During the above operation, the first rotation-transmitting member 85 is stationary, while the second axis Q.sub.1 is rotated counterclockwise through the angle .theta. to be brought to the Q.sub.11 position. As a result, a Y.sub.1 -side portion of the first connecting member 89 is wound around the first rotation-transmitting member 85, whereas a Y.sub.2 -side portion is unwound from the first rotation-transmitting member 85.
Thus, the first connecting member 89 is shifted in a direction shown by arrows a.sub.1 and a.sub.2. As a result, the second rotation-transmitting member 86 is rotated clockwise about the second axis Q.sub.1.
As previously mentioned, the radius ratio of the first rotation-transmitting member 85 to the second rotation-transmitting member 86 is 2 to 1. Thus, when the first arm 82 is rotated counterclockwise about the first axis P.sub.1 through the angle .theta., the second rotation-transmitting member 86 is rotated clockwise about the second axis Q.sub.11 through an angle 2.theta..
At this time, since the second rotation-transmitting member 86 is fixed to the second arm 83, the second rotation-transmitting member 86 and the second arm 83 are rotated clockwise about the second axis Q.sub.1 through an angle 2.theta..
If the second arm 83 is not moved relative to the first arm 82, the third axis is brought to the R.sub.11 position shown by broken lines when the first arm 82 is rotated counterclockwise about the first axis P.sub.1 through an angle .theta., starting from the initial state where the first, the second and the third axes P.sub.1, Q.sub.1, R.sub.1 are positioned on the same line. Actually, however, the second rotation-transmitting member 86 is rotated clockwise about the second axis Q.sub.11 through an angle 2.theta.. Therefore, the third axis R.sub.11 is rotated clockwise about the second axis Q.sub.11 through the angle 2.theta., and brought to the R.sub.12 position.
As a result, after the first arm 82 is rotated counterclockwise about the first axis P.sub.1 through an angle .theta., the third axis R.sub.12 is still on the straight line extending through the first and the third axis P.sub.1 and R.sub.1.
Further, when the second arm 83 is rotated clockwise about the second axis Q.sub.11 through an angle 2.theta. so that the third axis is brought to the R.sub.12 position, a Y.sub.2 -side portion of the second connecting member 90 is wound around the third rotation-transmitting member 87, whereas a Y.sub.1 -side portion is unwound from the third rotation-transmitting member 87.
As a result, the second connecting member 90 is shifted in a direction b.sub.1 -b.sub.2 shown in FIG. 16. Thus, the fourth rotation-transmitting member 88 is rotated counterclockwise about the third axis R.sub.12.
When the second arm 83 is rotated clockwise about the second axis Q.sub.11 through an angle 2.theta. as described above, the fourth rotation-transmitting member 88 is rotated counterclockwise about the third axis R.sub.12 through an angle .theta. since the radius ratio of the fourth rotation-transmitting member 88 to the third rotation-transmitting member 87 is 2 to 1. As a result, a point C.sub.0 of the fourth rotation-transmitting member 88 is brought to a point C.sub.1 on the straight line passing through the first and the third axes P.sub.1, R.sub.12.
Upon rotation of the first arm 82 about the first axis P.sub.1 in the counterclockwise direction as described above, the first arm mechanism 91 is actuated in the X-direction. Accordingly, the handling member 84A is moved along the line passing through the first and the third axes P.sub.1, R.sub.1. During this movement, however, the handling member 84A does not changed its attitude or orientation since it is fixed to the fourth rotation-transmitting member 88.
Likewise, the second arm mechanism 92 is actuated in the X-direction, while the second handling member 84B keeping its initial attitude along the line passing through the first and the third axes P.sub.2, R.sub.2.
The first and the second handling members 84A, 84B are arranged between the axes P.sub.1, P.sub.2 as viewed in the Y.sub.1 -Y.sub.2 direction. Further, the extremities of the handling members 84A, 84B are vertically spaced from each other. Thus, upon actuation of the arm mechanisms 91, 92, the handling members 84A, 84B can move along the X--X line passing through the axis O.sub.1 without interfering with each other.
When the inner frame 81 is rotated about the axis O.sub.1, the first and the second arm mechanisms 91, 92 are simultaneously rotated about the axis O.sub.1.
As shown in FIG. 17, a suitable number (six in the figure) of processing chambers are arranged around the axis O.sub.1 of the two-armed transfer robot. Workpieces are transferred by the robot to these chambers to be processed.
The prior art transfer robot has been found to have the following disadvantages. First, the fourth rotation-transmitting member 88 and the second connecting member 90 are provided at the extremity of the second arm 83 for maintaining the initial orientation of the handling member 84 along the P.sub.1 -R.sub.1 line. Therefore, the height H.sub.1 (see FIG. 15) of the arm mechanism is made unfavorably large. This requires that each processing chamber have a large insertion window to allow the passage of the arm mechanism.
Further, as shown in FIGS. 14-16, the axis P.sub.1 of the first arm mechanism 91 and the axis P.sub.2 of the second arm mechanism 92 are spaced from each other, with the axis O.sub.1 of the inner frame 81 located therebetween. This arrangement renders the rotation radius of the inner frame 81 unfavorably large.
Accordingly, the bearings 93 provided around the inner frame 81 have an unfavorable large diameter, and the magnetic fluid seal 94 for hermetic sealing suffers the same problem. With the use of such bearings and magnetic fluid seal, the overall size of the robot is also increased. Therefore, the price of the robot is rendered unduly high.
Further, the driving devices for linearly moving the handling members 84A, 84B are mounted on the inner frame 81. Thus, the driving devices are rotated together with the inner frame 81. For supplying the driving devices with electricity, use is made of a cable extending from the base frame 80. Thus, the rotation angle or the number of rotation of the inner frame 81 is limited to prevent the cable from breaking.
For realizing the above-mentioned prevention, a suitable monitoring device and a controlling unit are needed to stop the rotation of the inner frame 81 before the rotation angle of the frame exceeds a predetermined threshold value (540.degree. for example). However, such additional devices make the robot expensive. More importantly, the additional devices do not eliminate the limitation to the rotation angle. Thus, the conventional robot is not only expensive but inconvenient to operate.